Flow Through Animal Burrows in Mangrove Creeks

Mangrove swamp flats contain abundant burrows produced by crabs and other organisms. These burrows form a labyrinth of interconnected tubes through which water can flow. This flow may possibly provide an extremely efficient pathway for the transfer of nutrients, oxygen or other water chemistry, across the swamp-bed interface. A large percentage of crab burrows in the study area in North Queensland, Australia, have two or more openings. Dye injection studies, performed at the initial stages of inundation of the mangrove flats, showed that the water flowed through the burrows in the same direction as the surface current. The water slope driving the surface current must also cause a pressure difference between the two burrow openings, and is presumably responsible for the flow through the burrows. Flow velocities are very difficult to measure accurately but speeds of up to 30 mm s ^-1were recorded. A simple method for calculating the friction coefficient to flow and for the characteristic length for the burrow is presented. The method utilizes the period of, and decay in, water-level oscillations forced artificially in the burrow. Calculations of flow rates are also presented. The total flow through the animal burrows for a 1 km ²area of swamp in the study site was estimated conservatively to range from 1000 to 10 000 m ³per tidal cycle.